Method for producing a thermosetting polyurethane from a thermoplastic polyurethane and thermoset polyurethane obtainable using said method

ABSTRACT

A method for producing thermosetting polyurethane  
                 
 
     after transformation from thermoplastic polyurethane with a hydrolyzable organo-silane which is grafted onto macromolecules of said thermoplastic polyurethane by means of a bonding agent. Said organo-silane has general formula (I), wherein R 1 ,R 2 ,R 3 =aliphatic alkyl, aromatic alkyl; n&gt;1; R4 is an organic radical which can react with said bonding agent which comprises at least two isocyanate functions and the thermoplastic polyurethane obtained can, after transformation, become automatically cross-linked by coming into contact with water molecules in order to become thermosetting. The invention also relates to a product which can be obtained according to the inventive method.

[0001] The invention relates to a method for producing a polyurethanethat can be thermoset after processing, from a thermoplasticpolyurethane. It also relates to the polyurethane thermoset afterprocessing that can be obtained using said method.

[0002] Various thermoplastic materials are used to produce certainproducts, such as tubes for carrying hot fluids, electric cables, discwheels, seals, silentblocs, shoe soles, etc.

[0003] Thermoplastic materials are used in these different applications,particularly because they are easy to implement and have highperformance qualities at ambient temperature, are flexible and have ahigh degree of mechanical resistance. Nevertheless, these materials havethe drawback of low physical resistance to heat such that the productsobtained from said materials have, depending on their use, a shortservice life.

[0004] On the other hand, materials known as “thermosetting” materialsare difficult to shape, even though they may be heat resistant, suchthat their use is limited.

[0005] Faced with these various problems, the aim was therefore todevelop materials that combine the physical characteristics and ease ofprocessing of thermoplastic materials with the thermomechanicalcharacteristics of thermosetting materials, particularly heatresistance.

[0006] In order to achieve this, the preparation of a thermosettingpolymer has been proposed by modifying easily processed thermoplasticpolymers to enable the finished product to be cross-linked.

[0007] To achieve this aim a first method consists in mixing andextruding a polymer, particularly a polyethylene with a peroxide.However, this type of method not only has the drawback of being possiblewith a limited number of polyethylenes but also of requiring veryexpensive industrial installations.

[0008] Another method consists in irradiating a polymer with dosesmeasuring 80 to 200 KGy. It should, however, be noted that this type oftreatment is very expensive and also tends to deteriorate rather thanimprove the polymers used.

[0009] Another method, in particular that described in American patentsUS-A-3 648 155 and US-A-4 117 195, consists in grafting a hydrolysableorganosilane comprising at least one vinyl-type unsaturated chain in thepresence of a grafting initiator of vinyl functions, particularlyperoxide, and a condensation catalyst of the silane functions. Thehydrolysis and condensation of the silane functions enable the polymerto cross-link to obtain the thermoset state.

[0010] However, all the above methods only give satisfactory results onpolyethylene-based materials and their mixtures, copolymers orterpolymers as described for example in French patent FR-A-2 546 172.

[0011] The performances of polyethylene and its derivatives are notalways satisfactory for the planned applications: reduced heatresistance, reduced resistance to abrasion, reduced flexibility, etc.

[0012] Thermoplastic urethane (TPU) type polymers, which have verysatisfactory mechanical characteristics depending on the applicationsintended, are preferred to polyethylenes. All the standard known TPUs,however, lose their mechanical characteristics at around 70° C.Thermosetting polyurethanes can also be used even though they remaindifficult to work.

[0013] The problem posed therefore consists in developing amanufacturing procedure for producing polyurethane that maintains theprocessing conditions of thermoplastic urethane polymers (TPUs) whilepreserving the main mechanical characteristics and adding improved heatresistance that is greater than that of cross-linked polyethylenes.

[0014] In order to achieve this aim the applicant proposes a methodconsisting in grafting a hydrolysable organosilane comprising at leastone primary (—NH₂) or secondary (NH) amino chain directly onto athermoplastic polyurethane. However, in the temperature processingmethod the reaction with the amine causes the main macromolecular chainsto break. The silanes graft onto each end of the various fragmentsaccording to the reaction scheme below:

[0015] A grafted TPU is obtained that not only has a light molecularweight but also a low degree of thermomechanical resistance.

[0016] Faced with this drawback, the problem was therefore to propose anew method that enabled organosilanes to be grafted onto macromolecularchains of thermoplastic polyurethane that did not result indeteriorating said chains.

[0017] To achieve this the invention proposes a manufacturing method fora polyurethane that can be thermoset after processing using athermoplastic polyurethane according to which a cross-linking agent isused to graft a hydrolysable organosilane onto thermoplasticpolyurethane macromolecules, the hydrolysable organosilane having thegeneral formula:

[0018] where R₁, R₂, R₃=aliphatic alkyl, aromatic akyl; n≧1;

[0019] R₄ being an organic radical capable of reacting with the saidcross-linking agent which comprises at least two isocyanate functions,the thermoplastic polyurethane obtained being capable after processingof cross-linking on contact with water molecules to become thermoset.

[0020] In a preferred embodiment R₄ is selected from the groupcomprising the radicals NH₂, NH, SH, OH, phenol, epoxy. This list is notlimitative and R₄ is understood to be any organic radical capable ofreacting with an isocyanate function.

[0021] Advantageously the cross-linking agent is a diisocyanate with thegeneral formula:

O═C═N—R₅—N═C═O where R₅=organic radical

[0022] The applicant has found to its surprise that selecting bothmolecules of the organosilane type with R₄=NH₂, NH, SH, OH, phenol,epoxy and molecules of the isocyanate type with functionality greaterthan or equal to 2 made it possible effectively to graft organosilanesto the macromolecular chains of thermoplastic polyurethane withoutdamaging them.

[0023] The method of the invention has another advantage, viz. theability to adapt to all types of TPU including esters, ethers,carbonates and caprolactones.

[0024] In addition, the selected TPU may be aliphatic or aromatic.

[0025] Finally, it may be in amorphous or semi-crystalline form.

[0026] In an advantageous embodiment of the invention the organosilaneis aminopropyltrimethoxysilane, formula:

[0027] In this situation reactions between the urethane links of the TPUand isocyanate functions cause, either simultaneously or with a slighttime-lag depending on the mixing procedure, the formation ofaliophanates and isocyanate-amine reactions according to the reactionscheme below:

[0028] This series of reactions makes it possible to fix hydrolysablesilane groups onto the TPU chain without damaging it. The fact ofgrafting several silanes onto the same TPU chain can also encouragesubsequent cross-linking.

[0029] After processing, the thermoplastic polyurethane obtainedcross-links to the humidity by hydrolysis and polycondensation of thesilane functions grafted onto the various macro-molecular chains of theTPU in the classic silane hydrolysis and condensation reaction.

[0030] The diisocyanate used may advantageously be an aromatic,cycloaliphatic or aliphatic diisocyanate or their dimers.

[0031] The diisocyanate selected may advantageously be selected fromamong the following aromatic diisocyanates: TDI (1-3diisocyanatomethylbenzene), 2,4′-MDI (1 isocyanato-2(4-isocyanatophenyl)methylbenzene), 4,4′MDI (1,1-methylene bis (4-isocyanatobenzene)),2,4-TDI (2,4 diisocyanato-1-methylbenzene) or PPDI(1,4-diisocyanatobenzene) or their dimers.

[0032] The cycloaliphatic diisocyanate selected may advantageously beH₁₂ MDI (1,1-methylene bis (4-isocyanatocyclohexane)). Clearly the abovelist of diisocyanates that can be implemented in the method of theinvention is not exhaustive. The following can also be used: HDI(1,6-diisocyanatohexane), CHDI (trans-1,4-diisocyanatocyclohexane), IPDI(5-isocyanato-1-(isocyanatomethyl)-1,3,3-trimethylcyclo-hexane), TMDI(1,6-diisocyanato-2,2,4 (or 2,4,4)-trimethylhexane), m-TMXDI (1,3-bis(1-isocyanato-1-methylethylbenzene), p-TMXDI (1,4-bis(1-isocyanato-1-methylethylbenzene), NDI (1,5-diisocyanatonaphthalene),polymer MDI (isocyanic acid, polymethylene polyphenylene ester),Desmodur R (1,1′, 1″-methyllidynetris (4-isocyanotobenzene)), DesmodurRI (4-isocyanatophenol phosphorothioate (3:1) ester).

[0033] According to another aspect of the invention the concentration ofcross-linking agent required to manufacture TPU is between 0.1 and 30%by weight of TPU and advantageously between 3 and 4% by weight.

[0034] For a concentration lower than 1% by weight of TPU, the quantityof cross-linking agent is insufficient to avoid cutting the primarychains of TPU. This results in loss of thermo-mechanical properties ofthe cross-linked polymer.

[0035] For a concentration higher than 30% the results obtained are nobetter such that the method becomes financially less attractive.

[0036] At the same time, the concentration of organosilane required toobtain polyurethane that can be thermoset after processing is between0.05 and 15% of TPU by weight, and advantageously 2%.

[0037] For a concentration lower than 0.5% by weight, the mesh densityis insufficient to obtain an insoluble product.

[0038] For a concentration higher than 15% by weight the price of theingredients becomes financially less attractive.

[0039] In a first embodiment of the method of the invention:

[0040] a mixture of thermoplastic polyurethane and cross-linking agentis reacted at temperatures between 120 and 220° C. to produce a reagentTPU;

[0041] the reagent TPU mixture is then brought into contact with anorganosilane;

[0042] the resulting grafted TPU is then recovered.

[0043] The thermoplastic polyurethane obtained may subsequently begranulated or processed to produce sections of a given shape.

[0044] This type of reaction may be performed in one or two stages in avariety of reactors such as extruders, calenders, mixing tanks, etc.

[0045] When he method of the invention is implemented by extrusion asingle- or twin-screw machine is used the profile of which can be easilyadapted to each processed TPU. In this situation the extruder comprisesat least two mixing zones and at least three heating zones.

[0046] The maximum temperature applied during the extrusion process isbetween 120 and 220° C. depending on the type of TPU. This is introducedtogether with the molecule comprising at least one isocyanate function;the hydrolysable organosilane is then introduced into a zone locatedbetween 0 and 0.5 times the total length of the screw.

[0047] As explained above, the mixture to be extruded is granulated onremoval from the extruder or immmediately processed to obtain sectionsof a given shape. In the granular form the method has the advantage ofproviding half-finished products for use in other processes such asextrusion, calendering, injection, etc.

[0048] As explained above, half-finished TPU products that can bethermoset after processing may also be obtained by calendering or usinga Brabender or similar type internal mixer or any other processingmachine that shears the material at between 120 and 220° C.

[0049] In a second embodiment of the method of the invention:

[0050] the following are prepared separately:

[0051] a first mixture of thermoplastic polyurethane and cross-linkingagent at a temperature of between 20 and 60° C., and advantageously 40°C.;

[0052] a second mixture of thermoplastic polyurethane and organosilaneat a temperature of between 20 and 60° C.;

[0053] the two mixtures are caused to react with one another;

[0054] the resulting grafted TPU is then recovered.

[0055] As before, the thermoplastic polyurethane obtained is then eithergranulated or immediately processed.

[0056] The invention clearly relates to polyurethane that can bethermoset after processing and obtained by the method described above.

[0057] The invention and its advantages will be understood better fromthe following embodiments.

[0058]FIG. 1 is a chromatographic analysis of the grafted TPU of example1.

[0059]FIG. 2 shows changes in the Vicat point after cross-linking as afunction of the percentages of cross-linking agent and organosilane.

EXAMPLE 1

[0060] A mixture of TPU marketed by Goodrich under the trade-name Estane58201 together with 4 pcr of diTDI (toluene diisocyanate dimer), i.e. 4%diTDI by weight is introduced into the foot of the hopper of atwin-screw extruder. One-third of the way down the total length of thescrew 2.2 pcr of methylaminopropyl methoxysilane is introduced attemperatures of between 170 and 180° C.

[0061] The resulting product is stored in ambient air for two weeks.

[0062] The material is introduced into a TPU solvent; it is found thatmore than 85% of the material is insoluble, proving that cross-linkinghas indeed occurred and that the grafting has been effective.

[0063] Moreover, the chromatographic analysis of FIG. 1 shows that thereis no notable difference in molecular weight between standard TPU andgrafted TPU, thereby proving that the TPU chains have not been damaged.

EXAMPLE 2

[0064] In a single-screw extruder diameter 40 mm, length L/D 22, 3 pcrof MDI is introduced into the TPU marketed by Goodrich under thetrade-name Estane 58277 at 170° C. Granules are obtained.

[0065] The resulting granules are reintroduced into the same extrudertogether with 2 pcr of γ-APS (methoxysilane) at a temperature of 170° C.(these two operations can be performed in a single stage if the extruderis fitted with an incorporation system after the hopper).

EXAMPLE 3

[0066] TPU marketed by Goodrich under the trade-name Estane 58201 isintroduced into a mixer at a temperature of 170° C. After 30 seconds 2.5pcr of γ-APS (ethoxysilane) is introduced and 30 seconds after that 4pcr of TDI is added. The molten mass is then cooled.

EXAMPLE 4

[0067] As per Example 3 except that a calender heated to 120° C. is usedinstead of the mixer. TPU marketed by Goodrich under the trade-nameEstane 5715 is melted, then after 20 seconds 4 pcr of diTDI isintroduced and 30 seconds after that 2.2 pcr of M-APS (methoxysilane).The molten mass is then cooled.

[0068] In examples 2, 3 and 4 after a few days' exposure to humidity itwill be seen that more than 80% by weight of the materials is insolubleand the initial Vicat point of each material is increased by 40° C.

[0069]FIG. 1 showing the changes in the Vicat point as a function of thepercentage of isocyanate (molar ratio [organosilane/diisocyanate]=1)shows that for concentrations of diisocyanate greater than 4% the Vicatpoint of the original material is increased by 40° C.

[0070] The method makes it possible to retain the mechanicalcharacteristics of the original polymer at temperatures higher than 40°C.

[0071] The invention and its advantages are demonstrated by thedescription.

[0072] Of particular note is the simplicity of the method as it requiresno further stages after processing of thermoplastic polyurethane toobtain the cross-linking of the finished product.

1. Method for producing a polyurethane that can be thermoset afterprocessing, from a thermoplastic polyurethane in which a cross-linkingagent is used to graft a hydrolysable organosilane onto macromoleculesof thermoplastic polyurethane, the hydrolysable organosilane having thegeneral formula:

where R₁, R₂, R₃=aliphatic alkyl, aromatic akyl; n≧1; R₄ being anorganic radical capable of reacting with the said cross-linking agentwhich comprises at least two isocyanate functions, the thermoplasticpolyurethane obtained being capable after processing of cross-linking oncontact with water molecules to become capable of being thermoset. 2.Method as claimed in claim 1, characterised in that R₄ is selected fromthe group comprising the radicals NH₂, NH, SH, OH, phenol, epoxy. 3.Method as claimed in either of claims 1 or 2, characterised in that thecross-linking agent is a diisocyanate with the general formula:O═C═N—R₅—N═C═O where R₅=organic radical.
 4. Method as claimed in any ofthe foregoing claims, characterised in that the organosilane isaminopropyltrimethoxysilane, formula:


5. Method as claimed in any of the foregoing claims, characterised inthat the diisocyanate is selected from the group comprising aromatic,cycloaliphatic and aliphatic diisocyanates and their dimers.
 6. Methodas claimed in claim 5, characterised in that the aromatic diisocyanateis selected from the group including TDI (1-3diisocyanatomethylbenzene), 2,4′-MDI (1isocyanato-2(4-isocyanatophenyl)methylbenzene), 4,4′ MDI (1,1-methylenebis(4-isocyanatobenzene)), 2,4-TDI (2,4 diisocyanato-1-methylbenzene)and PPDI (1,4-diisocyanatobenzene) or their dimers.
 7. Method as claimedin claim 5, characterised in that the cycloaliphatic diisocyanate is H₁₂MDI (1,1-methylene bis(4-isocyanatocyclohexane)).
 8. Method as claimedin any of the foregoing claims, characterised in that the concentrationof cross-linking agent is between 0.1 and 30% by weight of TPU andadvantageously 4% by weight.
 9. Method as claimed in any of theforegoing claims, characterised in that the concentration oforganosilane is between 0.05 and 15% by weight of TPU, andadvantageously 2% by weight.
 10. Method as claimed in any of theforegoing claims, characterised in that: a mixture of thermoplasticpolyurethane and cross-linking agent is reacted at temperatures between120 and 220° C. to produce a reagent TPU; the reagent TPU mixture isthen brought into contact with an organosilane; the resulting graftedTPU is then recovered.
 11. Method as claimed in any of claims 1 to 9,characterised in that: the following are prepared separately: a firstmixture of thermoplastic polyurethane and cross-linking agent; a secondmixture of thermoplastic polyurethane and organosilane; the resultingmixture of the first and second mixtures is then extruded.
 12. Method asclaimed in either of claims 9 or 10, characterised in that the graftedthermoplastic polyurethane obtained is subsequently granulated orimmediately processed.
 13. Polyurethane capable of being thermoset afterprocessing obtained using the method described in any of claims 1 to 12.16. A polyurethane capable of being thermoset after processing obtainedusing the method of claim
 10. 17. A polyurethane capable of beingthermoset after processing obtained using the method of claim 11.